A conventional photovoltaic cell incorporates a semiconductor structure with a junction, such as a p-n junction formed with an n-type semiconductor and a p-type semiconductor. For the typical p-base configuration, a negative electrode is located on the side of the cell that is to be exposed to a light source (the “front” side, which in the case of a solar cell is the side exposed to sunlight), and a positive electrode is located on the other side of the cell (the “back” side). Radiation of an appropriate wavelength, such as sunlight, falling on the p-n junction serves as a source of external energy that generates electron-hole pair charge carriers. These electron-hole pair charge carriers migrate in the electric field generated by the p-n junction and are collected by electrodes on respective surfaces of the semiconductor. The cell is thus adapted to supply electric current to an electrical load connected to the electrodes, thereby providing electrical energy converted from the incoming solar energy that can do useful work.
Industrial photovoltaic cells are commonly provided in the form of a structure, such as one based on a doped crystalline silicon wafer, that has been metalized, i.e. provided with electrodes in the form of electrically conductive metal contacts through which the generated current can flow to an external electrical circuit load. Most commonly, these electrodes are provided on opposite sides of a generally planar cell structure. For the typical p-base configuration, the negative electrode is located on the front side of the cell; the positive electrode is located on the back side.
Both electrodes are conventionally produced by applying suitable conductive metal pastes to the respective surfaces of the semiconductor body and thereafter firing the pastes.
Photovoltaic cells are commonly fabricated with an insulating layer on their front side to afford an antireflective property that maximizes the utilization of incident light. However, in this configuration, the insulating layer normally must be removed to allow an overlaid front-side electrode to make contact with the underlying semiconductor surface. The front-side conductive metal paste typically include a fusible material, a conductive species (e.g., silver particles), and an organic vehicle or medium. The electrode may be formed by depositing the paste composition in a suitable pattern by screen printing and thereafter firing it to dissolve or otherwise penetrate the insulating layer and sinter the metal powder, such that an electrical connection with the underlying semiconductor structure is formed.
The ability of the paste composition to penetrate the anti-reflective coating and form a strong bond with the substrate upon firing is highly dependent on the composition of the conductive paste and firing conditions. Efficiency, a key measure of photovoltaic cell performance, is also influenced by the quality of the electrical contact made between the fired conductive ink and the substrate.
Although various methods and compositions useful in forming devices such as photovoltaic cells are known, there nevertheless remains a need for compositions that permit fabrication of patterned conductive structures that provide improved overall device electrical performance and that facilitate the efficient manufacture of such devices.